NROM memory cell, memory array, related devices and methods

ABSTRACT

An array of memory cells configured to store at least one bit per one F 2  includes substantially vertical structures providing an electronic memory function spaced apart a distance equal to one half of a minimum pitch of the array. The structures providing the electronic memory function are configured to store more than one bit per gate. The array also includes electrical contacts to the memory cells including the substantially vertical structures. The cells can be programmed to have one of a number of charge levels trapped in the gate insulator adjacent to the first source/drain region such that the channel region has a first voltage threshold region (Vt 1 ) and a second voltage threshold region (Vt 2 ) and such that the programmed cell operates at reduced drain source current.

RELATED APPLICATION

This Application is a Divisional of U.S. application Ser. No.10/738,408, titled “NROM MEMORY CELL, MEMORY ARRAY, RELATED DEVICES ANDMETHODS,” filed Dec. 16, 2003 issued May 22, 2007 as U.S. Pat. No.7,220,634, which is a continuation-in-part of U.S. application Ser. No.10/177,211, filed on Jun. 21, 2002, now abandoned, and acontinuation-in-part of U.S. application Ser. No. 10/232,411, filed onAug. 29, 2002, now abandoned, all of which are commonly assigned.

TECHNICAL FIELD

This invention relates to a NROM memory cells, arrays of such memorycells, electronic devices employing such memory cells and arrays, andmethods related to such memory cells.

BACKGROUND OF THE INVENTION

Various types of memory devices are used in electronic systems. Sometypes of memory device, such as DRAM (dynamic random access memory)provide large amounts of readable and writable data storage with modestpower budget and in favorably small form factor, but are not as fast asother types of memory devices and provide volatile data storagecapability. Volatile data storage means that the memory must becontinuously powered in order to retain data, and the stored data arelost when the power is interrupted. Nonvolatile memories are capable ofretaining data without requiring electrical power.

Other types of memory can provide read-only or read-write capabilitiesand non-volatile data storage, but are much slower in operation. Theseinclude CD-ROM devices, CD-WORM devices, magnetic data storage devices(hard discs, floppy discs, tapes and so forth), magneto-optical devicesand the like.

Still other types of memory provide very high speed operation but alsodemand high power budgets. Static RAM or SRAM is an example of suchmemory devices.

In most computer systems, different memory types are blended toselectively gain the benefits that each technology can offer. Forexample, read-only memories or ROM, EEPROM and the like are typicallyused to store limited amounts of relatively infrequently-accessed datasuch as a basic input-output system. These memories are employed tostore data that, in response to a power ON situation, configure aprocessor to be able to load larger amounts of software such as anoperating system from a high capacity non-volatile memory device such asa hard drive. The operating system and application software aretypically read from the high capacity memory and corresponding imagesare stored in DRAM.

As the processor executes instructions, some types of data may berepeatedly fetched from memory. As a result, some SRAM or other highspeed memory is typically provided as “cache” memory in conjunction withthe processor and may be included on the processor integrated circuit orchip and/or very near it.

Several different kinds of memory device are involved in most moderncomputing devices, and in many types of appliances that includeautomated and/or programmable features (home entertainment devices,telecommunications devices, automotive control systems etc.). As systemand software complexity increase, need for additional memory increases.Desire for portability, computation power and/or practicality result inincreased pressure to reduce both power consumption and circuit area perbit.

DRAMs have been developed to very high capacities in part because thememory cells can be manufactured to have a very small area, and thepower draw per cell can also be made quite small. In turn, this allowsmemory integrated circuits to be made that incorporate millions ofmemory cells in each chip. Typical one-transistor, one-capacitor DRAMmemory cells can be produced to have extremely small areal requirements.

Such areas are often equal to about 3 F×2 F, or less, where “F” isdefined as equal to one-half of minimum pitch (see FIG. 4, infra).Minimum pitch (i.e., “P”) is defined as equal to the smallest distanceof a line width (i.e., “W”) plus width of a space immediately adjacentthe line on one side of the line between the line and a next adjacentline in a repeated pattern within the array (i.e., “S”). Thus, in manyimplementations, the consumed area of a given DRAM cell is no greaterthan about 8 F².

However, because DRAMs are volatile memory devices, they require“refresh” operations. In a refresh operation, data are read out of eachmemory cell, amplified and written back into the DRAM. As a firstresult, the DRAM circuit is usually not available for other kinds ofmemory operations during the refresh operation. Additionally, refreshoperations are carried out periodically, resulting in times during whichdata cannot be readily extracted from or written to DRAMs. As a secondresult, some amount of electrical power is always needed to store datain DRAM devices.

As a third result, boot operations for computers such as personalcomputers involve a period during which the computer cannot be usedfollowing power ON initiation. During this period, operating systeminstructions and associated data, and application instructions andassociated data, are read from relatively slow, non-volatile memory,such as a conventional disc drive, are decoded by the processing unitand the resultant instructions and associated data are loaded intomodules incorporating relatively rapidly-accessible, but volatile,memory such as DRAM. Other consequences flow from the properties of thememory systems included in various electronic devices and theincreasingly complex software employed with them, however, theseexamples serve to illustrate ongoing needs.

Needed are methods and apparatus relating to non-volatile memoryproviding high areal data storage capacity, reprogrammability, low powerconsumption and relatively high data access speed.

SUMMARY OF THE INVENTION

In a first aspect, the present invention includes a method for making anarray of memory cells configured to store at least one bit per one F².The method includes doping a first region of a semiconductor substrateand incising the substrate to provide an array of substantially verticaledge surfaces. Pairs of the edge surfaces face one another and arespaced apart a distance equal to one half of a pitch of the array ofedges. The method also includes doping second regions between the pairsof edge surfaces and disposing respective structures each providing anelectronic memory function on at least some respective ones of the edgesurfaces. The method also includes establishing electrical contacts tothe first and second regions.

In another aspect, the present invention includes a method for making anarray of memory cells configured to store at least one bit per one F².The method includes disposing substantially vertical structuresproviding an electronic memory function spaced apart a distance equal toone half of a minimum pitch of the array and establishing electricalcontacts to memory cells including the vertical structures.

In a further aspect, the present invention includes an array of memorycells configured to store at least one bit per one F² formed usingvertical structures providing an electronic memory function spaced aparta distance equal to one half of a minimum pitch of the array. Thestructures providing the electronic memory function are configured tostore more than one bit per gate. The array also includes electricalcontacts to the memory cells including the vertical structures.

In a still further aspect, the present invention includes a verticalmetal oxide semiconductor field effect transistor (MOSFET) extendingoutwardly from a substrate, the MOSFET having a first source/drainregion, a second source/drain region, a channel region between the firstand the second source/drain regions, and a gate separated from thechannel region by a gate insulator. A sourceline is formed in a trenchadjacent to the vertical MOSFET, wherein the first source/drain regionis coupled to the sourceline. A transmission line is coupled to thesecond source/drain region. The can be programmed MOSFET to have one ofa number of charge levels trapped in the gate insulator adjacent to thefirst source/drain region such that the channel region has a firstvoltage threshold region (Vt1) and a second voltage threshold region(Vt2) and such that the programmed MOSFET operates at reduced drainsource current.

These and other embodiments, aspects, advantages, and features of thepresent invention will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the art byreference to the following description of the invention and referenceddrawings or by practice of the invention. The aspects, advantages, andfeatures of the invention are realized and attained by means of theinstrumentalities, procedures, and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below with reference to thefollowing accompanying drawings.

FIG. 1 is a simplified side view, in section, of a semiconductorsubstrate portion at one stage in processing, in accordance with anembodiment of the present invention.

FIG. 2 is a simplified side view, in section, of the substrate portionof FIG. 1 at a later stage in processing, in accordance with anembodiment of the present invention.

FIG. 3 is a simplified side view, in section, of the substrate portionof FIG. 2 at a later stage in processing, in accordance with anembodiment of the present invention.

FIG. 4 is a simplified plan view of a substrate portion showing aportion of a memory cell array, in accordance with an embodiment of thepresent invention.

FIG. 5 is a simplified side view, in section, illustrating arelationship between the structures of FIGS. 1-3 and the plan view ofFIG. 4, in accordance with an embodiment of the present invention.

FIG. 6 is a simplified plan view of a memory cell array illustrating aninterconnection arrangement for the memory cell array of FIG. 4, inaccordance with an embodiment of the present invention.

FIG. 7 is a simplified side view, in section, taken along section lines7-7 of FIG. 6, illustrating part of an interconnection arrangement inaccordance with an embodiment of the present invention.

FIG. 8 is a simplified side view, in section, taken along section lines8-8 of FIG. 6, illustrating part of an interconnection arrangement inaccordance with an embodiment of the present invention.

FIG. 9A is a block diagram of a metal oxide semiconductor field effecttransistor (MOSFET) in a substrate according to the teachings of theprior art.

FIG. 9B illustrates the MOSFET of FIG. 9A operated in the forwarddirection showing some degree of device degradation due to electronsbeing trapped in the gate oxide near the drain region over gradual use.

FIG. 9C is a graph showing the square root of the current signal (Ids)taken at the drain region of the conventional MOSFET versus the voltagepotential (VGS) established between the gate and the source region.

FIG. 10A is a diagram of a programmed MOSFET which can be used as amultistate cell in accordance with an embodiment of the presentinvention.

FIG. 10B is a diagram suitable for explaining the method by which theMOSFET of the multistate cell of the present invention can be programmedto achieve the embodiments of the present invention.

FIG. 10C is a graph plotting the current signal (Ids) detected at thedrain region versus a voltage potential, or drain voltage, (VDS) set upbetween the drain region and the source region (Ids vs. VDS) inaccordance with an embodiment of the present invention.

FIG. 11 illustrates a portion of a memory array in accordance with anembodiment of the present invention.

FIG. 12 illustrates an electrical equivalent circuit for the portion ofthe memory array shown in FIG. 11.

FIG. 13 is another electrical equivalent circuit useful in illustratinga read operation on the novel multistate cell in accordance with anembodiment of the present invention.

FIG. 14 illustrates a portion of a memory array in accordance with anembodiment of the present invention.

FIG. 15A, illustrates one embodiment of the gate insulator for thepresent invention having a number of layers, e.g., an ONO stack, wherethe layer closest to the channel includes an oxide layer, and a nitridelayer is formed thereon.

FIG. 15B aids to further illustrate the conduction behavior of the novelmultistate cell of the present invention.

FIG. 16A illustrates the operation and programming the novel multistatecell in the reverse direction.

FIG. 16B illustrates the now programmed multistate cell's operation inthe forward direction and differential read occurring in thisdifferential cell embodiment, e.g., 2 transistors in each cell.

FIG. 17 illustrates a memory device in accordance with an embodiment ofthe present invention.

FIG. 18 is a block diagram of an electrical system, or processor-basedsystem, utilizing a multistate cell constructed in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

The terms wafer and substrate used in the following description includeany structure having an exposed surface with which to form theintegrated circuit (IC) structure of the invention. The term substrateis understood to include semiconductor wafers. The term substrate isalso used to refer to semiconductor structures during processing, andmay include other layers that have been fabricated thereupon. Both waferand substrate include doped and undoped semiconductors, epitaxialsemiconductor layers supported by a base semiconductor or insulator, aswell as other semiconductor structures well known to one skilled in theart. The term conductor is understood to include semiconductors, and theterm insulator is defined to include any material that is lesselectrically conductive than the materials referred to as conductors.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

FIG. 1 is a simplified side view, in section, of a semiconductorsubstrate portion 20 at one stage in processing, in accordance with anembodiment of the present invention. The portion 20 includes etched orincised recesses 22, doped regions 24 and 26 and caps 28. The etchedrecesses 22 form trenches extending along an axis into and out of thepage of FIG. 1.

In one embodiment, the doped regions 24 are implanted n+ regions. In oneembodiment, the doped regions 24 are formed by a blanket implant. In oneembodiment, the caps 28 are dielectric caps and may be formed usingconventional silicon nitride and conventional patterning techniques. Inone embodiment, the etched recesses 22 are then etched usingconventional plasma etching techniques. In one embodiment, the dopedregions 26 are then doped by implantation to form n+ regions. The etchedor incised recesses 22 may be formed by plasma etching, laser-assistedtechniques or any other method presently known or that may be developed.In one embodiment, the recesses 22 are formed to have substantiallyvertical sidewalls relative to a top surface of the substrate portion20. In one embodiment, substantially vertical means at 90 degrees to thesubstrate surface, plus or minus ten degrees.

FIG. 2 provides a simplified side view, in section, of the substrateportion 20 of FIG. 1 at a later stage in processing, in accordance withan embodiment of the present invention. The portion 20 of FIG. 2includes thick oxide regions 32, ONO regions 34 formed on sidewalls 36of the recesses 22, gate material 38 and a conductive layer 40. In oneembodiment, the gate material 38 comprises conductively-dopedpolycrystalline silicon.

In one embodiment, conventional techniques are employed to oxidize thedoped regions 24 and 26 preferentially with respect to sidewalls 36. Asa result, the thick oxide regions 32 are formed at the same time as athinner oxide 42 on the sidewalls 36. These oxides also serve to isolatethe doped regions 24 and 26 from what will become transistor channelsalong the sidewalls 36. Other techniques for isolation may be employed.For example, in one embodiment, high density plasma grown oxides may beemployed. In one embodiment, spacers may be employed.

In one embodiment, conventional techniques are then employed to providea nitride layer 44 and an oxide layer 46, as is described, for example,in “NROM: A Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell”, byBoaz Eitan et al., IEEE Electron Device Letters, Vol. 21, No. 11,November 2000, pp. 543-545, IEEE Catalogue No. 0741-3106/00, or in “ATrue Single-Transistor Oxide-Nitride-Oxide EEPROM Device” by T. Y. Chanet al., IEEE Electron Device Letters, Vol. EDL-8, No. 3, March, 1987,pp. 93-95, IEEE Catalogue No. 0741-3106/87/0300-0093.

In one embodiment, the thin oxide 42, nitride layer 44 and oxide layer46 combine to form the ONO layer 34, such as is employed in SONOSdevices, while the polysilicon 38 forms a control gate. In operation,application of suitable electrical biases to the doped regions 24, 26and the control gate 38 cause hot majority charge carriers to beinjected into the nitride layer 44 and become trapped, providing athreshold voltage shift and thus providing multiple, alternative,measurable electrical states representing stored data. “Hot” chargecarriers are not in thermal equilibrium with their environment. In otherwords, hot charge carriers represent a situation where a population ofhigh kinetic energy charge carriers exist. Hot charge carriers may beelectrons or holes.

SONOS devices are capable of storing more than one bit per gate 38.Typically, the hot carriers are injected into one side 47 or 47′ of theONO layer 34, adjacent a contact, such as the region 24 or the region26, that provides a high electrical field.

By reversing the polarity of the potentials applied to the regions 24and 26, charge may be injected into the other side 47′ or 47 of the ONOlayer 34. Thus, four electronically-discriminable and distinct statescan be easily provided with a single gate 38. As a result, the structureshown in FIG. 2 is capable of storing at least four bits per gate 38.

FIG. 3 is a simplified side view, in section, of the substrate portion20 of FIG. 1 at an alternative stage in processing, in accordance withan embodiment of the present invention. The embodiment shown in FIG. 3includes the oxide regions 32 and 42, but a floating gate 48 is formedon the thin oxide region 42. A conventional oxide or nitride insulator49 is formed on the floating gate 48, followed by deposition of gatematerial 38. Floating gate devices are known and operate by injectinghot charge carriers, which may comprise electrons or holes, into thefloating gate 48.

Floating gate devices can be programmed to different charge levels thatcan be electrically distinct and distinguishable. As a result, it ispossible to program more data than one bit into each floating gatedevice, and each externally addressable gate 38 thus corresponds to morethan one stored bit. Typically, charge levels of 0, Q, 2Q and 3Q mightbe employed, where Q represents some amount of charge corresponding to areliably-distinguishable output signal.

FIG. 4 is a simplified plan view of a substrate portion showing aportion of a memory cell array 50, in accordance with an embodiment ofthe present invention. FIG. 4 also provides examples of pitch P, widthW, space S and minimum feature size F, as described in the Background.An exemplary memory cell area 52, the physical area of a singletransistor, can be seen to be about one F². Wordlines 54 are formed fromthe conductive layer 40, and bitlines 56 and 58 are formed.

FIG. 5 is a simplified side view, in section, illustrating arelationship between the structures of FIGS. 1-3 and the plan view ofFIG. 4, in accordance with an embodiment of the present invention. Thetrenches 22 correspond to bitlines 56 and 58, as is explained below inmore detail with reference to FIGS. 6-8.

The density of memory arrays such as that described with reference toFIGS. 1-5 can require interconnection arrangements that differ fromprior art memory arrays. One embodiment of a new type of interconnectionarrangement useful with such memory systems is described below withreference to FIGS. 6-8.

FIG. 6 is a simplified plan view illustrating an interconnectionarrangement 60 for the memory cell array 50 of FIG. 4, in accordancewith an embodiment of the present invention. The interconnectionarrangement 60 includes multiple patterned conductive layers 62 and 64,separated by conventional interlevel dielectric material 65 (FIGS. 7 and8). The views in FIG. 6-8 have been simplified to show correspondencewith the other Figures and to avoid undue complexity. Shallow trenchisolation regions 67 isolate selected portions from one another.

FIG. 7 is a simplified side view, in section, taken along section lines7-7 of FIG. 6, illustrating part of an interconnection arrangement inaccordance with an embodiment of the present invention.

FIG. 8 is a simplified side view, in section, taken along section lines8-8 of FIG. 6, illustrating part of an interconnection arrangement inaccordance with an embodiment of the present invention.

With reference to FIGS. 6-8, the patterned conductive layer 62 extendsupward to nodes 70, 70′, 70″ and establishes electrical communicationbetween the conductive layers 62 and selected portions of the dopedregion 24. The patterned conductive layer 62 stops at the line denoted72, 72′.

Similarly, other portions of the patterned conductive layer 62 extendfrom the line denoted 74, 74′ and extend upward, providing electricalcommunication from nodes 76, 76′, 76″ to other circuit elements. Thenodes 76, 76′, 76″ provide contact to selected portions of the dopedregion 24.

In contrast, patterned conductive layers 64 extend from top to bottom ofFIG. 6 and electrically couple to nodes 78, 78″ and thus to doped region26.

Such is but on example of a simplified interconnection arrangementsuitable for use with the memory devices of FIGS. 1-5. Otherarrangements are possible.

FIG. 9A is useful in illustrating the conventional operation of a MOSFETsuch as can be used in a DRAM array. FIG. 9A illustrates the normal hotelectron injection and degradation of devices operated in the forwarddirection. As is explained below, since the electrons are trapped nearthe drain they are not very effective in changing the devicecharacteristics.

FIG. 9A is a block diagram of a metal oxide semiconductor field effecttransistor (MOSFET) 101 in a substrate 100. The MOSFET 101 includes asource region 102, a drain region 104, a channel region 106 in thesubstrate 100 between the source region 102 and the drain region 104. Agate 108 is separated from the channel region 108 by a gate oxide 110. Asourceline 112 is coupled to the source region 102. A bitline 114 iscoupled to the drain region 104. A wordline 116 is coupled to the gate108.

In conventional operation, a drain to source voltage potential (Vds) isset up between the drain region 104 and the source region 102. A voltagepotential is then applied to the gate 108 via a wordline 116. Once thevoltage potential applied to the gate 108 surpasses the characteristicvoltage threshold (Vt) of the MOSFET a channel 106 forms in thesubstrate 100 between the drain region 104 and the source region 102.Formation of the channel 106 permits conduction between the drain region104 and the source region 102, and a current signal (Ids) can bedetected at the drain region 104.

In operation of the conventional MOSFET of FIG. 9A, some degree ofdevice degradation does gradually occur for MOSFETs operated in theforward direction by electrons 117 becoming trapped in the gate oxide110 near the drain region 104. This effect is illustrated in FIG. 9B.However, since the electrons 117 are trapped near the drain region 104they are not very effective in changing the MOSFET characteristics.

FIG. 9C illustrates this point. FIG. 9C is a graph showing the squareroot of the current signal (Ids) taken at the drain region versus thevoltage potential (VGS) established between the gate 108 and the sourceregion 102. The change in the slope of the plot of √{square root over(Ids)} versus VGS represents the change in the charge carrier mobilityin the channel 106.

In FIG. 9C, ΔVT represents the minimal change in the MOSFET's thresholdvoltage resulting from electrons gradually being trapped in the gateoxide 110 near the drain region 104, under normal operation, due todevice degradation. This results in a fixed trapped charge in the gateoxide 110 near the drain region 104. Slope 1 represents the chargecarrier mobility in the channel 106 for FIG. 9A having no electronstrapped in the gate oxide 110. Slope 2 represents the charge mobility inthe channel 106 for the conventional MOSFET of FIG. 9B having electrons117 trapped in the gate oxide 110 near the drain region 104. As shown bya comparison of slope 1 and slope 2 in FIG. 9C, the electrons 117trapped in the gate oxide 110 near the drain region 104 of theconventional MOSFET do not significantly change the charge mobility inthe channel 106.

There are two components to the effects of stress and hot electroninjection. One component includes a threshold voltage shift due to thetrapped electrons and a second component includes mobility degradationdue to additional scattering of carrier electrons caused by this trappedcharge and additional surface states. When a conventional MOSFETdegrades, or is “stressed,” over operation in the forward direction,electrons do gradually get injected and become trapped in the gate oxidenear the drain. In this portion of the conventional MOSFET there isvirtually no channel underneath the gate oxide. Thus the trapped chargemodulates the threshold voltage and charge mobility only slightly.

Applicant has previously described programmable memory devices andfunctions based on the reverse stressing of MOSFET's in a conventionalCMOS process and technology in order to form programmable address decodeand correction. (See generally, L. Forbes, W. P. Noble and E. H. Cloud,“MOSFET technology for programmable address decode and correction,” U.S.patent application Ser. No. 09/383,804). That disclosure, however, didnot describe multistate memory cell solutions, but rather address decodeand correction issues.

According to the teachings of the present invention, normal MOSFETs canbe programmed by operation in the reverse direction and utilizingavalanche hot electron injection to trap electrons in the gate oxide ofthe MOSFET. When the programmed MOSFET is subsequently operated in theforward direction the electrons trapped in the oxide are near the sourceand cause the channel to have two different threshold voltage regions.The novel programmed MOSFETs of the present invention conductsignificantly less current than conventional MOSFETs, particularly atlow drain voltages. These electrons will remain trapped in the gateoxide unless negative gate voltages are applied. The electrons will notbe removed from the gate oxide when positive or zero gate voltages areapplied. Erasure can be accomplished by applying negative gate voltagesand/or increasing the temperature with negative gate bias applied tocause the trapped electrons to be re-emitted back into the siliconchannel of the MOSFET. (See generally, L. Forbes, E. Sun, R. Alders andJ. Moll, “Field induced re-emission of electrons trapped in SiO₂,” IEEETrans. Electron Device, vol. ED-26, no. 11, pp. 1816-1818 (November1979); S. S. B. Or, N. Hwang, and L. Forbes, “Tunneling and Thermalemission from a distribution of deep traps in SiO₂,” IEEE Trans. onElectron Devices, vol. 40, no. 6, pp. 1100-1103 (June 1993); S. A. Abbasand R. C. Dockerty, “N-channel IGFET design limitations due to hotelectron trapping,” IEEE Int. Electron Devices Mtg., Washington D.C.,December 1975, pp. 35-38).

FIGS. 10A-10C are useful in illustrating the present invention in whicha much larger change in device characteristics is obtained byprogramming the device in the reverse direction and subsequently readingthe device by operating it in the forward direction.

FIG. 10A is a diagram of a programmed MOSFET which can be used as amultistate cell according to the teachings of the present invention. Asshown in FIG. 10A the multistate cell 201 includes a MOSFET in asubstrate 200 which has a first source/drain region 202, a secondsource/drain region 204, and a channel region 206 between the first andsecond source/drain regions, 202 and 204. In one embodiment, the firstsource/drain region 202 includes a source region 202 for the MOSFET andthe second source/drain region 204 includes a drain region 204 for theMOSFET. FIG. 10A further illustrates a gate 208 separated from thechannel region 206 by a gate oxide 210. A first transmission line 212 iscoupled to the first source/drain region 202 and a second transmissionline 214 is coupled to the second source/drain region 204. In oneembodiment, the first transmission line includes a sourceline 212 andthe second transmission line includes a bit line 214.

As stated above, multistate cell 201 is comprised of a programmedMOSFET. This programmed MOSFET has a charge 217 trapped in the gateoxide 210 adjacent to the first source/drain region 202 such that thechannel region 206 has a first voltage threshold region (Vt1) and asecond voltage threshold region (Vt2) in the channel 206. In oneembodiment, the charge 217 trapped in the gate oxide 210 adjacent to thefirst source/drain region 202 includes a trapped electron charge 217.According to the teachings of the present invention and as described inmore detail below, the multistate cell can be programmed to have one ofa number of charge levels trapped in the gate insulator adjacent to thefirst source/drain region 202 such that the channel region 206 will havea first voltage threshold region (Vt1) and a second voltage thresholdregion (Vt2) and such that the programmed multistate cell operates atreduced drain source current.

FIG. 10A illustrates the Vt2 in the channel 206 is adjacent the firstsource/drain region 202 and that the Vt1 in the channel 206 is adjacentthe second source/drain region 204. According to the teachings of thepresent invention, Vt2 has a higher voltage threshold than Vt1 due tothe charge 217 trapped in the gate oxide 217 adjacent to the firstsource/drain region 202. Multiple bits can be stored on the multistatecell 201.

FIG. 10B is a diagram suitable for explaining the method by which theMOSFET of the multistate cell 201 of the present invention can beprogrammed to achieve the embodiments of the present invention. As shownin FIG. 10B the method includes programming the MOSFET in a reversedirection. Programming the MOSFET in the reverse direction includesapplying a first voltage potential V1 to a drain region 204 of theMOSFET. In one embodiment, applying a first voltage potential V1 to thedrain region 204 of the MOSFET includes grounding the drain region 204of the MOSFET as shown in FIG. 10B. A second voltage potential V2 isapplied to a source region 202 of the MOSFET. In one embodiment,applying a second voltage potential V2 to the source region 202 includesapplying a high positive voltage potential (VDD) to the source region202 of the MOSFET, as shown in FIG. 10B. A gate potential VGS is appliedto a gate 208 of the MOSFET. In one embodiment, the gate potential VGSincludes a voltage potential which is less than the second voltagepotential V2, but which is sufficient to establish conduction in thechannel 206 of the MOSFET between the drain region 204 and the sourceregion 202. As shown in FIG. 10B, applying the first, second and gatepotentials (V1, V2, and VGS respectively) to the MOSFET creates a hotelectron injection into a gate oxide 210 of the MOSFET adjacent to thesource region 202. In other words, applying the first, second and gatepotentials (V1, V2, and VGS respectively) provides enough energy to thecharge carriers, e.g. electrons, being conducted across the channel 206that, once the charge carriers are near the source region 202, a numberof the charge carriers get excited into the gate oxide 210 adjacent tothe source region 202. Here the charge carriers become trapped.

In one embodiment of the present invention, the method is continued bysubsequently operating the MOSFET in the forward direction in itsprogrammed state during a read operation. Accordingly, the readoperation includes grounding the source region 202 and precharging thedrain region a fractional voltage of VDD. If the device is addressed bya wordline coupled to the gate, then its conductivity will be determinedby the presence or absence of stored charge in the gate insulator. Thatis, a gate potential can be applied to the gate 208 by a wordline 216 inan effort to form a conduction channel between the source and the drainregions as done with addressing and reading conventional DRAM cells.

However, now in its programmed state, the conduction channel 206 of theMOSFET will have a first voltage threshold region (Vt1) adjacent to thedrain region 204 and a second voltage threshold region (Vt2) adjacent tothe source region 202, as explained and described in detail inconnection with FIG. 10A. According to the teachings of the presentinvention, the Vt2 has a greater voltage threshold than the Vt1 due tothe hot electron injection 217 into a gate oxide 210 of the MOSFETadjacent to the source region 202.

FIG. 10C is a graph plotting a current signal (Ids) detected at thesecond source/drain region 204 versus a voltage potential, or drainvoltage, (VDS) set up between the second source/drain region 204 and thefirst source/drain region 202 (Ids vs. VDS). In one embodiment, VDSrepresents the voltage potential set up between the drain region 204 andthe source region 202. In FIG. 10C, the curve plotted as D1 representsthe conduction behavior of a conventional MOSFET which is not programmedaccording to the teachings of the present invention. The curve D2represents the conduction behavior of the programmed MOSFET, describedabove in connection with FIG. 10A, according to the teachings of thepresent invention. As shown in FIG. 10C, for a particular drain voltage,VDS, the current signal (IDS2) detected at the second source/drainregion 204 for the programmed MOSFET (curve D2) is significantly lowerthan the current signal (IDS1) detected at the second source/drainregion 204 for the conventional MOSFET which is not programmed accordingto the teachings of the present invention. Again, this is attributed tothe fact that the channel 206 in the programmed MOSFET of the presentinvention has two voltage threshold regions and that the voltagethreshold, Vt2, near the first source/drain region 202 has a highervoltage threshold than Vt1 near the second source/drain region due tothe charge 217 trapped in the gate oxide 217 adjacent to the firstsource/drain region 202.

Some of these effects have recently been described for use in adifferent device structure, called an NROM, for flash memories. Thislatter work in Israel and Germany is based on employing charge trappingin a silicon nitride layer in a non-conventional flash memory devicestructure. (See generally, B. Eitan et al., “Characterization of ChannelHot Electron Injection by the Subthreshold Slope of NROM device,” IEEEElectron Device Lett., Vol. 22, No. 11, pp. 556-558, (November 2001); B.Etian et al., “NROM: A novel localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Lett., Vol. 21, No. 11, pp. 543-545,(November 2000)). Charge trapping in silicon nitride gate insulators wasthe basic mechanism used in MNOS memory devices (see generally, S. Sze,Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506),charge trapping in aluminum oxide gates was the mechanism used in MIOSmemory devices (see generally, S. Sze, Physics of Semiconductor Devices,Wiley, N.Y., 1981, pp. 504-506), and Applicant has previously disclosedcharge trapping at isolated point defects in gate insulators (seegenerally, L. Forbes and J. Geusic, “Memory using insulator traps,” U.S.Pat. No. 6,140,181, issued Oct. 31, 2000).

In contrast to the above work, the present invention disclosesprogramming a MOSFET in a reverse direction to trap one of a number ofcharge levels near the source region and reading the device in a forwarddirection to form a multistate memory cell based on a modification ofDRAM technology.

Prior art DRAM technology generally employs silicon oxide as the gateinsulator. Further the emphasis in conventional DRAM devices is placedon trying to minimize charge trapping in the silicon oxide gateinsulator. According to the teachings of the present invention, avariety of insulators are used to trap electrons more efficiently thanin silicon oxide. That is, in the present invention, the multistatememory cell employs charge trapping in gate insulators such as, wetsilicon oxide, silicon nitride, silicon oxynitride SON, silicon richoxide SRO, aluminum oxide Al₂O₃, composite layers of these insulatorssuch as oxide and then silicon nitride, or oxide and then aluminumoxide, or multiple layers as oxide-nitride-oxide. While the chargetrapping efficiency of silicon oxide may be low such is not the case forsilicon nitride or composite layers of silicon oxide and nitride.

FIG. 11 illustrates a portion of a memory array 300 according to theteachings of the present invention. The memory in FIG. 11 is shownillustrating a number of vertical pillars, or multistate cells, 301-1and 301-2 formed according to the teachings of the present invention. Asone of ordinary skill in the art will appreciate upon reading thisdisclosure, the number of vertical pillars are formed in rows andcolumns extending outwardly from a substrate 303. As shown in FIG. 11,the number of vertical pillars, 301-1 and 301-2 are separated by anumber of trenches 340. According to the teachings of the presentinvention, the number of vertical pillars, 301-1 and 301-2, serve astransistors including a first source/drain region, 302-1 and 302-2,respectively. The first source/drain region, 302-1 and 302-2, is coupledto a sourceline 304. As shown in FIG. 11, the sourceline 304 is formedin a bottom of the trenches 340 between rows of the vertical pillars,301-1 and 301-2. In one embodiment, according to the teachings of thepresent invention, the sourceline 304 is formed from a doped regionimplanted in the bottom of the trench. A second source/drain region,306-1 and 306-2 respectively, is coupled to a bitline (not shown). Achannel region 305 is located between the first and the secondsource/drain regions.

As shown in FIG. 11, a gate 309 is separated from the channel region 305by a gate insulator 307 in the trenches 340 along rows of the verticalpillars, 301-1 and 301-2. In one embodiment, according to the teachingsof the present invention, the gate insulator 307 includes a gateinsulator 307 selected from the group of silicon dioxide (SiO₂) formedby wet oxidation, silicon oxynitride (SON), silicon rich oxide (SRO),and aluminum oxide (Al₂O₃). In another embodiment, according to theteachings of the present invention, the gate insulator 307 includes agate insulator 307 selected from the group of silicon rich aluminumoxide insulators, silicon rich oxides with inclusions of nanoparticlesof silicon, silicon oxide insulators with inclusions of nanoparticles ofsilicon carbide, and silicon oxycarbide insulators. In anotherembodiment, according to the teachings of the present invention, thegate insulator 307 includes a composite layer 307. In this embodiment,the composite layer 307 includes a composite layer 307 selected from thegroup of an oxide-aluminum oxide (Al₂O₃)-oxide composite layer, andoxide-silicon oxycarbide-oxide composite layer. In another embodiment,the composite layer 307 includes a composite layer 307, or anon-stoichiometric single layer, of two or more materials selected fromthe group of silicon (Si), titanium (Ti), and tantalum (Ta). In anotherembodiment, according to the teachings of the present invention, thegate insulator 307 includes an oxide-nitride-oxide (ONO) gate insulator307.

FIG. 12 illustrates an electrical equivalent circuit 400 for the portionof the memory array shown in FIG. 11. As shown in FIG. 12, a number ofvertical multistate cells, 401-1 and 401-2, are provided. Each verticalmultistate cell, 401-1 and 401-2, includes a first source/drain region,402-1 and 402-2, a second source/drain region 406-1 and 406-2, a channelregion 405 between the first and the second source/drain regions, and agate 409 separated from the channel region by a gate insulator 407.

FIG. 12 further illustrates a number of bit lines, 411-1 and 411-2,coupled to the second source/drain region 406-1 and 406-2 of eachmultistate cell. In one embodiment, as shown in FIG. 12, the number ofbit lines, 411-1 and 411-2, are coupled to the second source/drainregion 406-1 and 406-2 along rows of the memory array. A number of wordlines, such as wordline 413 in FIG. 12, are coupled to the gate 409 ofeach multistate cell along columns of the memory array. And, a number ofsourcelines, such as common sourceline 415, are coupled to the firstsource/drain regions, e.g. 402-1 and 402-2, along columns of thevertical multistate cells, 401-1 and 401-2, such that adjacent pillarscontaining these transistors share the common sourceline 415. In oneembodiment, column adjacent pillars include a transistor which operatesas a vertical multistate cell, e.g. 401-1, on one side of a sharedtrench, the shared trench separating rows of the pillars as described inconnection with FIG. 11, and a transistor which operates as a referencecell, e.g. 401-2, having a programmed conductivity state on the oppositeside of the shared trench. In this manner, according to the teachings ofthe present invention and as described in more detail below, at leastone of multistate cells can be programmed to have one of a number ofcharge levels trapped in the gate insulator, shown generally as 417,adjacent to the first source/drain region, e.g. 402-1, such that thechannel region 405 will have a first voltage threshold region (Vt1) anda second voltage threshold region (Vt2) and such that the programmedmultistate cell operates at reduced drain source current.

FIG. 13 is another electrical equivalent circuit useful in illustratinga read operation on the novel multistate cell 500 according to theteachings of the present invention. The electrical equivalent circuit inFIG. 13 represents a programmed vertical multistate cell. As explainedin detail in connection with FIG. 11, the programmed vertical multistatecell 500 includes a vertical metal oxide semiconductor field effecttransistor (MOSFET) 500 extending outwardly from a substrate. The MOSFEThas a source region 502, a drain region 506, a channel region 505between the source region 502 and the drain region 506, and a gate 509separated from the channel region 505 by a gate insulator, showngenerally as 507.

As shown in FIG. 13 a wordline 513 is coupled to the gate 509. Asourceline 504, formed in a trench adjacent to the vertical MOSFET asdescribed in connection with FIG. 11, is coupled to the source region502. A bit line, or data line 511 is coupled to the drain region 506.The multistate cell 500 shown in FIG. 13 is an example of a programmedmultistate cell 500 having one of a number of charge levels trapped inthe gate insulator, shown generally as 517, adjacent to the firstsource/drain region, 502, such that the channel region 505 will have afirst voltage threshold region (Vt1) and a second voltage thresholdregion (Vt2) and such that the programmed multistate cell 500 operatesat reduced drain source current. According to the teachings of thepresent invention, the second voltage threshold region (Vt2) is now ahigh voltage threshold region which is greater than the first voltagethreshold region (Vt1).

FIG. 14 illustrates a portion of a memory array 600 according to theteachings of the present invention. The memory in FIG. 14 is shownillustrating a pair of multistate cells 601-1 and 601-2 formed accordingto the teachings of the present invention. As one of ordinary skill inthe art will understand upon reading this disclosure, any number ofmultistate cells can be organized in an array, but for ease ofillustration only two are displayed in FIG. 14. As shown in FIG. 14, afirst source/drain region, 602-1 and 602-2 respectively, is coupled to asourceline 604. A second source/drain region, 606-1 and 606-2respectively, is coupled to a bitline, 608-1 and 608-2 respectively.Each of the bitlines, 608-1 and 608-2, couple to a sense amplifier,shown generally at 610. A wordline, 612-1 and 612-2 respectively, iscouple to a gate, 614-1 and 614-2 respectively, for each of themultistate cells, 601-1 and 601-2. According to the teachings of thepresent invention, the wordlines, 612-1 and 612-2, run across or areperpendicular to the rows of the memory array 600. Finally, a writedata/precharge circuit is shown at 624 for coupling a first or a secondpotential to bitline 608-1. As one of ordinary skill in the art willunderstand upon reading this disclosure, the write data/prechargecircuit 624 is adapted to couple either a ground to the bitline 608-1during a write operation in the reverse direction, or alternatively toprecharge the bitline 608-1 to fractional voltage of VDD during a readoperation in the forward direction. As one of ordinary skill in the artwill understand upon reading this disclosure, the sourceline 604 can bebiased to a voltage higher than VDD during a write operation in thereverse direction, or alternatively grounded during a read operation inthe forward direction.

As shown in FIG. 14, the array structure 600, including multistate cells601-1 and 601-2, has no capacitors. Instead, according to the teachingsof the present invention, the first source/drain region or sourceregion, 602-1 and 602-2, are coupled directly to the sourceline 604. Inorder to write, the sourceline 604 is biased to voltage higher than VDDand the devices stressed in the reverse direction by grounding the dataor bit line, 608-1 or 608-2. If the multistate cell, 601-1 or 601-2, isselected by a word line address, 612-1 or 612-2, then the multistatecell, 601-1 or 601-2, will conduct and be stressed with accompanying hotelectron injection into the cells gate insulator adjacent to the sourceregion, 602-1 or 602-2. As one of ordinary skill in the art willunderstand upon reading this disclosure, a number of different chargelevels can be programmed into the gate insulator adjacent to sourceregion such that the cells is used as a differential cell and/or thecell is compared to a reference or dummy cell, as shown in FIG. 14, andmultiple bits can be stored on the multistate cell.

During read the multistate cell, 601-1 or 601-2, is operated in theforward direction with the sourceline 604 grounded and the bit line,608-1 or 608-2, and respective second source/drain region or drainregion, 606-1 and 606-2, of the cells precharged to some fractionalvoltage of Vdd. If the device is addressed by the word line, 612-1 or612-2, then its conductivity will be determined by the presence orabsence of the amount of stored charge trapped in the gate insulator asmeasured or compared to the reference or dummy cell and so detectedusing the sense amplifier 610. The operation of DRAM sense amplifiers isdescribed, for example, in U.S. Pat. Nos. 5,627,785; 5,280,205; and5,042,011, all assigned to Micron Technology Inc., and incorporated byreference herein. The array would thus be addressed and read in theconventional manner used in DRAM's, but programmed as multistate cellsin a novel fashion.

In operation the devices would be subjected to hot electron stress inthe reverse direction by biasing the sourceline 604, and read whilegrounding the sourceline 604 to compare a stressed multistate cell, e.g.cell 601-1, to an unstressed dummy device/cell, e.g. 601-2, as shown inFIG. 14. The write and possible erase feature could be used duringmanufacture and test to initially program all cells or devices to havesimilar or matching conductivity before use in the field. Likewise, thetransistors in the reference or dummy cells, e.g. 601-2, can allinitially be programmed to have the same conductivity states. Accordingto the teachings of the present invention, the sense amplifier 610 canthen detect small differences in cell or device characteristics due tostress induced changes in device characteristics during the writeoperation.

As one of ordinary skill in the art will understand upon reading thisdisclosure such arrays of multistate cells are conveniently realized bya modification of DRAM technology. According to the teachings of thepresent invention a gate insulator of the multistate cell includes gateinsulators selected from the group of thicker layers of SiO₂ formed bywet oxidation, SON silicon oxynitride, SRO silicon rich oxide, Al₂O₃aluminum oxide, composite layers and implanted oxides with traps (L.Forbes and J. Geusic, “Memory using insulator traps,” U.S. Pat. No.6,140,181, issued Oct. 31, 2000). Conventional transistors for addressdecode and sense amplifiers can be fabricated after this step withnormal thin gate insulators of silicon oxide.

FIGS. 15A-15B and 16A-16B are useful in illustrating the use of chargestorage in the gate insulator to modulate the conductivity of themultistate cell according to the teachings of the present invention.That is, FIGS. 15A-16B illustrate the operation of the novel multistatecell 701 formed according to the teachings of the present invention. Asshown in FIG. 15A, the gate insulator 707 has a number of layers, e.g.an ONO stack, where layer 707A is the oxide layer closest to the channel705 and a nitride layer 707B is formed thereon. In the embodiment shownin FIG. 15A the oxide layer 707A is illustrated having a thickness ofapproximately 6.7 nm or 67 Å (roughly 10⁻⁶ cm). In the embodiment shownin FIG. 15A a multistate cell is illustrated having dimensions of 0.1 μm(10⁻⁵ cm) by 0.1 μm. For purposes of illustration, the charge storageregion near the source can reasonably have dimensions of 0.1 micron(1000 Å) by 0.02 micron (200 Å) in a 0.1 micron technology. If the gateoxide 707A nearest the channel 705 is 67 Å then a charge of 100electrons will cause a threshold voltage shift in this region of 1.6Volts since the oxide capacitance is about 0.5 micro-Farad (μF) persquare centimeter. If the transistor has a total effective oxidethickness of 200 Å then a change in the threshold voltage of only 0.16Volts near the source, corresponding to 10 electrons, is estimated tochange the transistor current by 4 micro Amperes (μA). The senseamplifier described in connection with FIG. 14, which is similar to aDRAM sense amplifier, can easily sense this charge difference on thedata or bitlines. In this embodiment, the sensed charge difference onthe data or bitlines will be 40 femto Coulombs (fC) over a sense periodof 10 nano seconds (nS).

To illustrate these numbers, the capacitance, Ci, of the structuredepends on the dielectric constant, ∈i, (which for silicon dioxide SiO₂equates to 1.06/3×10⁻¹² F/cm), and the thickness of the insulatinglayers, t, (given here as 6.7×10⁻⁷ cm), such that Ci=∈i/t=((1.06×10⁻¹²F/cm/(3×6.7×10⁻⁷ cm))=0.5×10⁻⁶ Farads/cm² (F/cm²). This value taken overthe charge storage region near the source, e.g. 20 nm×100 nm or 2×10⁻¹¹cm², results in a capacitance value of Ci=10⁻¹⁷ Farads. Thus, for achange in the threshold voltage of ΔV =1.6 Volts the stored charge mustbe Q=C×ΔV=(10⁻¹⁷ Farads×1.6 Volts)=1.6×10⁻¹⁷ Coulombs. Since Q=Nq, thenumber of electrons stored is approximately Q/q=(1.6×10⁻¹⁷Coulombs/1.6×10⁻¹⁹ Coulombs) or 100 electrons. In effect, the programmedmultistate cell, or modified MOSFET is a programmed MOSFET having acharge trapped in the gate insulator adjacent to a first source/drainregion, or source region, such that the channel region has a firstvoltage threshold region (Vt1) and a second voltage threshold region(Vt2), where Vt2 is greater than Vt1, and Vt2 is adjacent the sourceregion such that the programmed MOSFET operates at reduced drain sourcecurrent. For ΔQ=100 electrons in the dimensions given above, if thetransistor has a total effective oxide thickness of 200 Å then a changein the threshold voltage of only 0.16 Volts near the source,corresponding to 10 electrons, is estimated to change the transistorcurrent by 4 micro Amperes (μA). As stated above, the sense amplifierdescribed in connection with FIG. 14, which is similar to a DRAM senseamplifier, can easily sense this charge difference on the data orbitlines. And, the sensed charge difference on the data or bitlines willbe 40 femto Coulombs (fC) over a sense period of 10 nano seconds (nS)for this representative one of a number of stored charge levelsaccording to the teachings of the present invention. Again, a number ofdifferent charge levels can be programmed into the gate insulatoradjacent to source region such that the cell is used as a differentialcell and/or the cell is compared to a reference or dummy cell, as shownin FIG. 14, and multiple bits can be stored on the multistate cell ofthe present invention.

FIG. 15B aids to further illustrate the conduction behavior of the novelmultistate cell of the present invention. The electrical equivalentcircuit shown in FIG. 15B illustrates a multistate cell 701 having anequivalent oxide thickness of 200 Å. The charge storage region near thesource 702 can reasonably have a length dimension of 0.02 micron (20 nm)in a 0.1 micron technology with a width dimension of 0.1 micron (100nm). Therefore, for a change in the drain source voltage (ΔVDS) in thisregion an electric field of E=(0.1 V/2×10⁻⁶ cm)=0.5×105 V/cm or 5×104V/cm is provided. The drain current is calculated using the formulaID=μCox×(W/L)×(Vgs−Vt)×ΔVDS. In this example, μCox=μCi is taken as 50μA/V2 and W/L=5. Appropriate substitution into the drain currentprovides ID=(50 μA/V2×5×0.16 Volts×0.1 Volts)=2.5×1.6 μA=4 μA. As notedabove this drain current ID corresponds to 10 electrons trapped in thegate insulator, or charge storage region 707 near the source 702. Sensedover a period of 10 nanoseconds (nS) produces a current on the bitlineof 40 fC (e.g. 4 μA×10 nS=40×10⁻¹⁵ Coulombs).

FIGS. 16A and 16B, illustrate the operation and programming the novelmultistate cell as described above. However, FIGS. 16A and 16B also helpillustrate an alternative array configuration where adjacent devices arecompared and one of the devices on the opposite side of a shared trenchis used as a dummy cell transistor or reference device. Again, thereference devices can all be programmed to have the same initialconductivity state. FIG. 16A illustrates the operation and programmingthe novel multistate cell in the reverse direction. As shown in FIG.16A, a transistor 801-1 on one side of the trench (as described inconnection with FIG. 11) is stressed by grounding its respective drainline, e.g. 811-1. As shown in FIG. 16A, the drain line 811-2 for thetransistor 801-2 on the opposite side of the trench is left floating. Avoltage is applied to the shared sourceline 804 located at the bottom ofthe trench (as described in connection with FIG. 11) which now acts as adrain. As shown in this electrical equivalent circuit, the neighboring(shared trench)/column adjacent transistors, 801-1 and 801-2, share agate 807 and the wordline 813, e.g. polysilicon gate lines, couplingthereto run across or are perpendicular to the rows containing the bitand source lines, e.g. 811-1, 811-2, and 804. A gate voltage is appliedto the gates 807. Here the multistate cell 801-1 will conduct and bestressed with accompanying hot electron injection into the cells gateinsulator 817 adjacent to the source region 802-1.

FIG. 16B illustrates the now programmed multistate cell's operation inthe forward direction and differential read occurring in a thisdifferential cell embodiment, e.g. 2 transistors in each cell. To readthis state the drain and source (or ground) have the normal connectionsand the conductivity of the multistate cell is determined. That is, thedrain line, 811-1 and 811-2, have the normal forward direction potentialapplied thereto. The shared sourceline 804 located at the bottom of thetrench (as described in connection with FIG. 11) is grounded and onceagain acts as a source. And, a gate voltage is applied to the gates 807.As one of ordinary skill in the art will understand upon reading thisdisclosure, a number of different charge levels can be programmed intothe gate insulator 817 adjacent to source region 802-1 and compared tothe reference or dummy cell, 802-2. Thus, according to the teachings ofpresent invention multiple bits can be stored on the multistate cell.

As stated above, these novel multistate cells can be used in a DRAM likearray. Two transistors can occupy an area of 4 F squared (F=the minimumlithographic feature size) when viewed from above, or each memory cellconsisting of one transistor utilizing an area of 2 F squared. Eachtransistor can now, however, store many bits so the data storage densityis much higher than one bit for each 1 F squared unit area. Using areference or dummy cell for each memory transistor where the referencetransistor is in close proximity, e.g. the embodiment shown in FIGS. 16Aand 16B vs. that shown in FIG. 12, results in better matchingcharacteristics of transistors, but a lower memory density.

In FIG. 17 a memory device is illustrated according to the teachings ofthe present invention. The memory device 940 contains a memory array942, row and column decoders 944, 948 and a sense amplifier circuit 946.The memory array 942 consists of a plurality of multistate cells 900,formed according to the teachings of the present invention whose wordlines 980 and bit lines 960 are commonly arranged into rows and columns,respectively. The bit lines 960 of the memory array 942 are connected tothe sense amplifier circuit 946, while its word lines 980 are connectedto the row decoder 944. Address and control signals are input onaddress/control lines 961 into the memory device 940 and connected tothe column decoder 948, sense amplifier circuit 946 and row decoder 944and are used to gain read and write access, among other things, to thememory array 942.

The column decoder 948 is connected to the sense amplifier circuit 946via control and column select signals on column select lines 962. Thesense amplifier circuit 946 receives input data destined for the memoryarray 942 and outputs data read from the memory array 942 overinput/output (I/O) data lines 963. Data is read from the cells of thememory array 942 by activating a word line 980 (via the row decoder944), which couples all of the memory cells corresponding to that wordline to respective bit lines 960, which define the columns of the array.One or more bit lines 960 are also activated. When a particular wordline 980 and bit lines 960 are activated, the sense amplifier circuit946 connected to a bit line column detects and amplifies the conductionsensed through a given multistate cell, where in the read operation thesource region of a given cell is couple to a grounded array plate (notshown), and transferred its bit line 960 by measuring the potentialdifference between the activated bit line 960 and a reference line whichmay be an inactive bit line. The operation of Memory device senseamplifiers is described, for example, in U.S. Pat. Nos. 5,627,785;5,280,205; and 5,042,011, all assigned to Micron Technology Inc., andincorporated by reference herein.

FIG. 18 is a block diagram of an electrical system, or processor-basedsystem, 1000 utilizing multistate memory cells 1012 constructed inaccordance with the present invention. That is, the multistate memorycells 1012 utilizes the modified DRAM cell as explained and described indetail in connection with FIGS. 2-4. The processor-based system 1000 maybe a computer system, a process control system or any other systememploying a processor and associated memory. The system 1000 includes acentral processing unit (CPU) 1002, e.g., a microprocessor, thatcommunicates with the multistate memory 1012 and an I/O device 1008 overa bus 1020. It must be noted that the bus 1020 may be a series of busesand bridges commonly used in a processor-based system, but forconvenience purposes only, the bus 1020 has been illustrated as a singlebus. A second I/O device 1010 is illustrated, but is not necessary topractice the invention. The processor-based system 1000 can alsoincludes read-only memory (ROM) 1014 and may include peripheral devicessuch as a floppy disk drive 1004 and a compact disk (CD) ROM drive 1006that also communicates with the CPU 1002 over the bus 1020 as is wellknown in the art.

It will be appreciated by those skilled in the art that additionalcircuitry and control signals can be provided, and that the memorydevice 1000 has been simplified to help focus on the invention. At leastone of the multistate cell in NROM 1012 includes a programmed MOSFEThaving a charge trapped in the gate insulator adjacent to a firstsource/drain region, or source region, such that the channel region hasa first voltage threshold region (Vt1) and a second voltage thresholdregion (Vt2), where Vt2 is greater than Vt1, and Vt2 is adjacent thesource region such that the programmed MOSFET operates at reduced drainsource current.

It will be understood that the embodiment shown in FIG. 18 illustratesan embodiment for electronic system circuitry in which the novel memorycells of the present invention are used. The illustration of system1000, as shown in FIG. 18, is intended to provide a generalunderstanding of one application for the structure and circuitry of thepresent invention, and is not intended to serve as a completedescription of all the elements and features of an electronic systemusing the novel memory cell structures. Further, the invention isequally applicable to any size and type of memory device 1000 using thenovel memory cells of the present invention and is not intended to belimited to that described above. As one of ordinary skill in the artwill understand, such an electronic system can be fabricated insingle-package processing units, or even on a single semiconductor chip,in order to reduce the communication time between the processor and thememory device.

Applications containing the novel memory cell of the present inventionas described in this disclosure include electronic systems for use inmemory modules, device drivers, power modules, communication modems,processor modules, and application-specific modules, and may includemultilayer, multichip modules. Such circuitry can further be asubcomponent of a variety of electronic systems, such as a clock, atelevision, a cell phone, a personal computer, an automobile, anindustrial control system, an aircraft, and others.

CONCLUSION

Utilization of a modification of well established DRAM technology andarrays will serve to afford an inexpensive memory device which can beregarded as disposable if the information is later transferred toanother medium, for instance CDROM's. The high density of DRAM arraystructures will afford the storage of a large volume of digital data orimages at a very low cost per bit. There are many applications where thedata need only be written a limited number of times, the low cost ofthese memories will make it more efficient to just utilize a new memoryarray, and dispose of the old memory array, rather than trying to eraseand reuse these arrays as is done with current flash memories. The novelmultistate cells can be used in a DRAM like array. Two transistors canoccupy an area of 4 F squared (F=the minimum lithographic feature size)when viewed from above, or each memory cell consisting of one transistorutilizing an area of 2 F squared. Each such transistor can now, however,store many bits so the data storage density is much higher than one bitfor each 1 F squared unit area. Using a reference or dummy cell for eachmemory transistor where the reference transistor is in close proximity,e.g., the embodiment shown in FIGS. 16A and 16B vs. that shown in FIG.12, results in better matching characteristics of transistors, but alower memory density.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. An array of memory cells configured to store at least one bit per oneF² comprising: memory cells arranged in rows and columns each coupled torespective row and column decoding circuitry, wherein each memory cellcomprises: first doped regions formed on a surface of a semiconductorsubstrate; an array of incisions formed into the substrate to provide anarray of substantially vertical edge surfaces, pairs of the edgesurfaces facing one another as sidewalls of a trench and spaced apart adistance equal to one half of a pitch of the array of edge surfaces;second doped regions formed only between the pairs of edge surfaces atthe bottom of the trench; respective ONO structures each providing anelectronic memory function, without an additional floating gate,disposed only on the vertical edge surfaces such that a nitride layer ofthe ONO structure is formed only on the vertical edge surfaces and alower oxide layer of the ONO structure is formed, without the nitridelayer, on the surface of the semiconductor substrate over the firstdoped regions and on the bottom of the trenches; a shared gate formedbetween the ONO structures; and electrical contacts to the first andsecond regions and to the structures providing the electronic memoryfunction.
 2. The array of claim 1 wherein the structures providing anelectronic memory function each comprise structures each configured tostore more than one bit per gate.
 3. The array of claim 1 wherein thesemiconductor substrate comprises silicon.
 4. An array of memory cellsconfigured to store at least one bit per one F² comprising: memory cellsarranged in rows and columns each coupled to respective row and columndecoding circuitry, wherein each memory cell comprises: substantiallyvertical ONO structures providing an electronic memory function, withouta floating gate, spaced apart a distance equal to one half of a minimumpitch of the array to form sidewalls of a trench such that a pair oftrenches form a pillar; a drain region formed in the top of the pillarwherein a nitride layer of the ONO structure is formed only on thesidewalls of each trench and an oxide layer of the ONO structure isformed, without the nitride layer, over each pillar and on the bottom ofeach trench; a source region formed in the bottom of each trench; ashared gate formed between the vertical structures; and electricalcontacts to the memory cells including the substantially verticalstructures.
 5. The array of claim 4 wherein the electrical contactsinclude electrical contacts to the drain and source regions and to thesubstantially vertical structures.
 6. The array of claim 5 wherein thesubstantially vertical ONO structures each comprise: structuresproviding the electronic memory function configured to store more thanone bit per gate.
 7. The array of claim 5 wherein the substantiallyvertical structures each include respective polysilicon gates.
 8. Thearray of claim 4 wherein the structures providing the electronic memoryfunction are configured to store more than one bit per gate.
 9. Thearray of claim 4 wherein the substantially vertical structures compriserespective polysilicon gates.
 10. The array of claim 4 wherein thesubstantially vertical structures are configured to provide anelectronic memory function by storing holes.